Field emission displays (FEDs) are a type of thin, lightweight, flat panel information display. These displays are, in effect, flat cathode ray tubes that use matrix-addressed cold cathodes to produce light from a cathodoluminescent phosphor screen. FEDs consists of a field emission array, dielectric spacers, and a phosphor-coated (monochrome or color) faceplate with matrix-addressable electronics. The field emission array comprises electron emitters, each smaller than an individual pixel, that might employ gate electrodes. The electron emitter material may be shaped in any geometrical configuration (e.g. shaft tip, line edge, plane, etc.). Electrons are emitted into a vacuum when an electric field of sufficient strength is applied to the emitter material. The electrons are accelerated to an electron target such as the phosphor-coated screen. The phosphor then luminesces and the pixel "turns on".
FEDs employ high voltage spacers, typically comprising dielectric materials such as ceramics, glass, or high temperature plastics to separate the emitting plate from the phosphor plate. The spacing between the emitter and the phosphor is very small (about 1-10 mm) and is critical to optimal display performance. The spacers must meet several requirements, such as high dielectric strength, resistance to surface flashover, low secondary electron emission, low leakage current, ability to dissipate electrostatic charge, and good mechanical strength. In addition, these materials must maintain these properties under high energy electron bombardment for extended periods. In operation, many dielectric materials are prone to surface flashover, dielectric breakdown, and poor electronic control. It has been exceedingly difficult in the field to find a material which meets the above requirements, especially the control of secondary electron emission and charging.
Dielectric spacers are used in field emission displays (FEDs) to separate the anode faceplate (screen) from the cathode material. Preferably, such spacers must possess a high dielectric strength (greater than about 10.sup.6 V/cm), high electrical resistance (from about 10.sup.+8 to about 10.sup.+11 ohm-cm), high resistance to flashover, good thermal conductivity and resistance to arcing damage. Furthermore, the structural and chemical properties of the spacers must not change throughout the operational lifetime of the display (greater than about 10,000 hours).
Presently, dielectric spacers are most commonly made from bulk substrate materials, such as glass and ceramics. These materials satisfy the FEDs' dielectric strength requirements but have limited ranges of electrical resistivity and have secondary electron emission coefficients (SEEC) typically much greater than unity (greater than 1.0), for example 2.0 to 3.5. Primary electron refer to electrons from a source, such as an electron beam, which impact a substrate surface. Secondary electron emission refers to the electrons which are emitted from a substrate surface after being impacted by primary electrons. The secondary electron emission coefficient (SEEC) is a ratio value representing the average number of secondary electrons emitted from a bombarded substrate surface for every incident primary electron on the substrate surface.
A material which meets the dielectric strength requirements for desired electrical applications, including use with FEDs, and which also has electrical resistivity values that can be predictably altered, or "tuned", while also having a SEEC value of less than unity (less than 1.0), is presently unknown, but would be advantageous. The present invention relates to the unexpected results that the present coatings are much thinner than those known and provide a low secondary electron emission coefficient of less than about 1.0, while maintaining all other desirable properties, and providing for high productivity and lower cost.
Such a material as described above would also benefit other applications. Color picture tubes use either perforated shadow masks or grilles with vertical slits to direct electron trajectory to an electron target, typically a phosphor coated screen. Electrons from the tube's electron guns pass through the mask or grille and are directed at slightly different angles to excite a red, blue, or green phosphor. Precise alignment of the electron beams is required to achieve sharp images with high contrast. Some fraction of the electrons typically fall on the mask or grille and generate secondary electrons. This may result in defocusing of the image-forming beam due to its interaction with the secondary electrons which have uncontrolled trajectories. Higher resolution images and enhanced brightness and contrast can be achieved if the production of secondary electrons is suppressed or eliminated.
Carbon-containing coatings have been applied to electrical components that are bombarded by electrons. Carbon has many distinct phases, for example, diamond, graphite, soot, etc. Each of these carbon phases has a different secondary electron emission coefficient, or SEEC, for example diamond=2.8; graphite=1.0; and soot=0.45. Certain applications, including electronic displays or other component parts incorporated into electronics under vacuum, require coatings or substrate materials having a SEEC of a specified value. Many electronics applications require coatings having extremely low SEEC values, for example, &lt;1.0 in combination with other properties such as durability, adhesion and smoothness. Certain C:H and Si:C thin films have been attempted for use with high frequency waveguides. Such films as reported by Groudeva-Zotova et al. (Diamond and Related Materials, Vol. 5, Nov. 10, 1987), have low SEEC values in the energy range of from 250-2000 eV. The SEEC on these films is very sensitive to film composition and morphology. Also they must be annealed to lower the SEEC. Finally, the electrical resistivity cannot be tailored. In addition, coatings containing graphite in the form of Aquadag (Acheson Colloids, Port Huron, Mich.), vacuum pyrolyzed graphite, and lamp black deposited by electrophoresis, have been used on high frequency electronic devices to prevent multi-pactor discharges (surface flashover). However, these films often must be applied at paint thicknesses of from 10 .mu.m to over 100 .mu.m. This creates adhesion problems and other limitations adversely affecting electrical tailorability, durability, stability and smoothness. Further, U.S. Pat. No. 5,466,431 discloses a 0.5 to 2.0 micron thick two network nanocomposite film having a high thermal conductivity and low secondary emission used as a protective coating on the grids of color TV tubes. However, such thick coatings are not only unnecessary, but are also disadvantageous for display applications. Coatings at such thicknesses have a high cost, lower overall productivity due to long deposition times, and low equipment efficiency. Such a thick film coating may also cause variations in critical physical dimensions of the substrate.
As a result, low SEEC coatings which can be applied at required thicknesses and which have no adhesion problem are not known. Coatings for electronic components, especially FEDs and cathode ray tubes, which have both a low SEEC (of less than about unity, i.e. less than about 1.0) and which have superior adhesion and are electrically tunable over a broad range would be highly advantageous.